Avoid LOCK/UNLOCK hard-coding in gc_locks.h for PS3 target

Avoid LOCK/UNLOCK hard-coding in gc_locks.h for PS3 target

* include/private/gc_locks.h (USE_PTHREAD_LOCKS): Define for
SN_TARGET_PS3 target.
* include/private/gcconfig.h (NO_PTHREAD_TRYLOCK): Likewise.
* include/private/gc_locks.h (GC_allocate_ml, LOCK, UNLOCK): Remove
explicit definition for SN_TARGET_PS3.
* include/private/gc_locks.h (UNCOND_LOCK): Define as
pthread_mutex_lock(...) instead of GC_lock() (which calls
pthread_mutex_lock) if USE_PTHREAD_LOCKS but not GC_ASSERTIONS and
not USE_SPIN_LOCK.